Think Out Loud

What tiny worms found in decades-old cans of salmon reveal about marine ecosystems

By Sheraz Sadiq (OPB)
April 23, 2024 12 p.m.

Broadcast: Wednesday, April 24

In the spring of 2021, Natalie Mastick, a graduate student in the University of Washington’s School of Aquatic and Fishery Sciences, dissected filets from nearly 200 cans of salmon, some of which were more than 40 years old. The cans contained filets from pink, chum, coho and sockeye salmon that were caught in Alaska between 1979 and 2021. Mastick acquired the cans from the archives of the Seafood Products Association, a trade organization based in Seattle that provides quality control testing to seafood processors.


Mastick and her colleagues opened up the cans to look for the presence of a parasitic roundworm embedded inside the filets. The parasite requires multiple hosts, including salmon, to complete its life cycle. Its abundance can serve as an indicator of the health of the marine food web, and how infections are changing over time for species like salmon and killer whales. Mastick is now a postdoctoral associate of student programs at the Yale Peabody Museum. She joins us to talk about her recently published findings.

Note: The following transcript was created by a computer and edited by a volunteer.

Dave Miller: This is Think Out Loud on OPB. I’m Dave Miller. A few years ago, a graduate student at the University of Washington opened some very old cans of salmon. They spanned from the Carter administration, 1979, to the Biden administration. Natalie Mastick is a marine mammal researcher, meaning she focuses on some of the largest animals on the planet. But as she opened can after can of decades-old salmon, she was looking for something a lot smaller: tiny parasitic roundworms. And she found them. She joins us now to talk about this work. Natalie, welcome.

Natalie Mastick: Thank you so much.

Miller: So as I just noted, you are a marine mammal researcher. Why is it that you devoted a chunk of your time to these tiny worms?

Mastick: So I was interested in studying the health of killer whales in Puget Sound, specifically southern resident killer whales, which you might have heard of because they’re pretty endangered. And we know quite a bit about some of the threats to killer whales, including not enough salmon in the ocean, a lot of loud ships going by, a lot of toxic contaminants. And I was interested in thinking about what threats they might have to their health that we can’t see. What if those salmon they were eating came at a cost? And that’s how I started thinking about the worms that infect these whales.

Miller: Can you describe the life cycle of these worms? And how do you pronounce the name of it?

Mastick: These are anisakid nematodes.

Miller: So what is the life cycle of these worms? And why are they important?

Mastick: These nematodes infect all kinds of marine mammals. They start out their lives as a little egg in the ocean. That egg then hatches, and hopefully for the parasite, gets eaten by a small zooplankton. Like a copepod or a krill. And then that krill or copepod gets eaten by a small fish, which hopefully then gets eaten by a larger fish, and then eventually gets eaten by a marine mammal.

In each of those intermediate hosts, the parasite is growing a little bit more and developing into a different life stage. And then when it reaches the marine mammal host, it sets up shop. It wants to be there for the rest of its life. It mates and reproduces, and sheds a bunch of eggs. And then those eggs get shed back out into the ocean through the whale or seal or sea lion’s poop. And then those eggs can start the cycle over again by hatching and getting eaten.

Miller: So these worms will only reproduce if they end up in the marine mammal at the end, at the end of that food chain?

Mastick: Yes, they need the marine mammal to be able to survive to a point where they can reproduce.

Miller: So, in a sense, they could be seen as a proxy. If you find worms, it’s a sign that there were whales or other marine mammals?

Mastick: Yeah, they’re pretty closely tied to the abundance of marine mammals in an area.

Miller: How did you get the idea to look into – I was going to say ancient, they’re not ancient, but in terms of canned food maybe that word is appropriate. Cans of salmon that are more than 40 years old, how did you get the idea to look in those?

Mastick: When I started my PhD, I was really interested in seeing how parasitized – so how big of infections whales have today – and how that’s changed over time, if that’s a new problem or not. It’s really hard to find old whales, they take up a lot of room. It’s very unlikely that somebody would see a whale that washes up on shore and then put it in a freezer, and have enough of those available for me to actually look for trends over time. So we used fish as a proxy for whale infection risk, the likelihood that these whales would be getting infected at a given time. And the lab that I was working at UW, Chelsea Wood’s lab, we did this with museum fish quite often. We would look inside fish that were preserved in ethanol in natural history museums. And we would dissect those, and find all of the parasites that live inside of those fish.

So I really wanted to do that for salmon as well. But salmon, similar to marine mammals, also take up a lot of space. There aren’t a lot of adult salmon in natural history museum collections. So I was looking for another source of data to answer this question when this seafood inspection company reached out to my advisor Chelsea, and said, “Hey, we have these cans of salmon that we’ve been collecting since the seventies in our basement. Would you like to look in them for parasites?” And this seemed like it could be like the missing dataset that I’ve been looking for.

So we gave it a shot, even though opening ancient cans of fish sounded pretty daunting.

Miller: Before we get to that, why is it that they had this archive or library of really old cans of salmon? Why do they store them to begin with?

Mastick: The Seafood Products Association, they’re basically a seafood quality and control industry in Seattle. Before cans of salmon go to market, they will inspect a subset of those. And they’ll do this by looking at them and smelling them for quality. And then a subset of those cans, they figured that they would hang on to, to see how the cans degrade over time, how the cans themselves hold up. They just thought maybe these could come in handy someday. And they absolutely did.

Miller: It’s fascinating to me that they reached out to you, to your team, as opposed to the other way around. They knew what you and the scientists you work with were interested in studying, and they said “hey, we’ve got some salmon you might want to look at from 1979?”

Mastick: Yeah. The group that I work with, we talk about parasites a lot. And we kind of got a reputation in the Seattle area for looking for parasites in weird places. So they had heard about our work through I think the Seattle Aquarium, and reached out to us. Which was just an amazing call to get.

Miller: I should say, we have talked with Chelsea Wood before about marine parasites. And she is a kind of Pied Piper of excitement about parasites. So I can see why the Seafood Products Association would have been excited to say “hey, we’ve got some samples you might want to look at.”

What is it like to open a can of salmon from 1979 or 1983?

Mastick: It’s smelly, that’s for sure. These cans definitely have degraded quite a bit with time. Cans of salmon usually are shelf stable for five years, so we were definitely pushing that limit. But we did our can opening essentially randomly. We never knew what year of can we were opening, which kind of helped with the daunting nature of opening a 40 year old can.

Miller: Could you basically tell though? If there was one from the year before, or one from 41 years before, was it pretty obvious that there was a huge time difference?


Mastick: Yeah, you could. We tried not to let that influence our counts at all. But you could definitely tell. It looked more appetizing if it was like a year old versus 40 years old.

Miller: Are you somebody who in the past would sometimes eat canned salmon?

Mastick: I have been vegetarian since I was 15. So, no, I have never eaten canned salmon before.

Miller: Did that make this even more, maybe unappetizing is the wrong word because that was never going to be the issue. But did it make this more distasteful, to be opening these cans of ancient fish meat?

Mastick: That’s an interesting question. I don’t think so. I’m pretty well versed in dissecting gross things and smelly things. I’ve participated in quite a few marine mammal necropsies. One of my projects is looking at whale poop. This was kind of just another day at work.

Miller: So you open up a can, and then what would you do?

Mastick: We had to figure out how to find the worms in the cans. A postdoc in our lab, Rachel Welicky, and I, we did a lot of method testing to figure out how to best find these worms. And in the past, parasitologists will use this method called candling. If you have a fresh filet of fish, you can press it between two pieces of glass and then shine a flashlight behind it, and all of the worms show up. You can see where they’re encysted in the fish’s muscle. But as you can imagine, the transparency of raw salmon is a lot different than cooked salmon. So we tried that with the cooked salmon, and we couldn’t find them at all.

We ended up actually picking through every single one of these filets with two pairs of tweezers. We would just pick it apart into really small pieces. The worms kind of encyst in the muscle of the fish, and they form these little pockets. So those pockets get pretty easy to detect when you’re used to looking at them really closely, and when you pick them apart into small enough pieces. It’s things that you wouldn’t notice if you were just prepping some canned salmon to eat. But if you are looking for them, they’re pretty obvious.

Miller: How big are these worms?

Mastick: They’re about a centimeter long, a little bit longer.

Miller: Half an inch or something? That’s not small.

Mastick: You can see them with your naked eye. They do kind of blend in with the filet itself. They’re often a similar color, and they kind of mimic the text of muscle filament in the fish. So it would be really easy to miss them if you didn’t know what to look for.

Miller: And maybe just as well to miss them.

My understanding is that these are harmless when cooked. They could give you food poisoning if they’re raw. How common was it, over all the samples, to find these worms in these cans?

Mastick: In about half of the cans that we opened we found worms. We ended up opening and dissecting 178 cans. That percentage varied by the species of salmon that we were looking at. And it also varied how many worms that we would find. Some of them had just one. There was one can that we opened that had 115 worms, but that was definitely an anomaly. Most of them, it was under 10.

Miller: But that makes me think that there is a good chance if people have canned salmon from Alaska in their pantries right now, that there’s a pretty good chance that there are worms in those cans.

Mastick: Yeah. They don’t work on picking out these worms for the canned salmon industry. In fresh and frozen filets, they will pick out most of the worms that they can find. But because these are filets that were destined to be cooked, and cooked worms don’t have an impact on the consumer, they weren’t screened for that.

Miller: You found an increase in worms over time in two of the species you looked at, in pink and chum salmon. What’s your hypothesis for what happened over that time?

Mastick: As we kind of talked about at the beginning, these worms are pretty closely tied to marine mammals. So marine mammals need to be in the area for these worms to be able to finish out their life cycle and reproduce. So our study period, which was the duration of time that we had cans, so from 1979 to 2021, that’s directly following a major piece of legislation that happened in the US, which was the Marine Mammal Protection Act in 1973. And after that protection act was passed, all of the marine mammals in Alaska were protected from hunting and culling. And in a lot of those populations, we saw increases in abundance for whales and dolphins and seals and sea lions in Alaska, as well as all over the US.

And so we’re seeing this increase in marine mammals, and then we see this increase in their parasites following it. So my main hypothesis is that this increase in the number of these marine mammal hosts in Alaska released a bottleneck on these parasite populations, and they were able to increase as well.

Miller: What do you see as the limits of this particular methodology, looking at worms in canned salmon. What can you tell from this and what can’t you tell?

Mastick: There are quite a few limitations. We were really limited by the cans that we had available. As you can imagine, beggars can’t be choosers when it comes to inheriting four decades of canned salmon. We had really been hoping to look at this trend in Washington to see what infections look like in salmon over time in Washington State. Those data didn’t exist. So we switched to Alaska, because we still thought that was an interesting question.

And then there are things about the fish themselves that we can’t know. We don’t know where exactly they were caught. We know where they were canned because that data exists. We don’t know what kind of vessel they were caught on, or if that vessel had refrigeration, which could influence the amount of parasites in the filet. So there are definitely limitations in what we can find out about the fish that we dissected, which is a problem with all historical ecological studies. We always wish that people would have collected better data in the past than they did. You can never know what people are going to be using your collection for later on. And nobody expected us to be looking at parasites in fish that were caught and canned for commercial markets.

Miller; What’s next? What do you want to learn next to further these findings?

Mastick: I’m interested in seeing how this plays out in marine mammals today. I have been working on a project where we’re assessing how parasitized killer whales are in Alaska, Washington, and Vancouver, with these parasites. And we have found that these parasite infections are pretty common in these whales. So that increase that we’re seeing in the salmon certainly seems to be reflecting the health of the whales themselves.

We also don’t really know what kind of effect these parasites have on whale health, because we only ever get to look at whale health in captivity or when we’re dissecting a dead whale, which is not exactly representative of a live, healthy population.

I also really like doing these types of projects where we get to look in unexpected places to find answers, especially about parasites. There are parasites everywhere, whether we like to think about them or not. So it’d be interesting to see if anyone has a collection of, I don’t know, tuna or sardines or anchovies, similar to this one. Or if there are other sources of historical data on parasites in fish that we could look at to better tease out these patterns in ecosystem change.

Miller: Natalie, thanks very much.

Mastick: Thank you. This was fun.

Miller: I agree. Natalie Mastick is a postdoctoral associate at the Yale Peabody Museum. She joined us to talk about the work she did as a graduate student at the University of Washington.

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